Method for interpretation of seismic records to yield indication of gaseous hydrocarbons

ABSTRACT

The present invention indicates that gas-containing strata of an earth formation have low Poisson&#39;s ratios and that the acoustic contrast with the overburden rock has a surprising effect as a function of the angle of incidence on a seismic wave associated with an array of sources and detectors: viz., a significant-and progressive-change in P-wave reflection coefficient as a function of the angle of incidence occurs. Thus, differentiating between high-intensity amplitude anomalies of nongas- and gas-containing media is simplified: progressive change in amplitude intensity of resulting traces generated by the field array as a function of offset between each source-detector pair, is associated with the last-mentioned medium only.

This is a continuation of application Ser. No. 756,584, filed Jan. 3,1977, now abandoned.

FIELD OF THE INVENTION

The present invention pertains to the art of seismic prospecting forpetroleum reservoirs by multiple-point surveying techniques, and moreparticularly to the art of converting high-intensity reflectionamplitude anomalies associated with one or more common centerpointsobserved on seismic record traces into diagnostic indicators of thepresence of gas in the underlying subsurface strata.

BACKGROUND OF THE INVENTION

For several decades, seismic prospecting for petroleum has involved thecreation of acoustic disturbances above, upon, or just below the surfaceof the earth, using explosives, air guns, or large mechanical vibrators.Resulting acoustic waves propagate downwardly in the earth, and arepartially reflected back toward the surface when acoustic impedancechanges within the earth are encountered. A change from one rock type toanother, for example, may be accompanied by an acoustic impedancechange, so that the reflectivity of a particular layer depends on thevelocity and density content between that layer and the layer whichoverlies it, say according to the formula ##EQU1## where AR is theamplitude from the reflected signal and Ai is the amplitude of theincident signal; V₁ is the velocity of the wave in the overlying medium1; V₂ is the velocity in the medium layer below the contact line; d₁ isthe density of the overlying medium 1; and d₂ is the density of theunderlying medium.

In early years, signal traces of the reflected acoustic waves wererecorded immediately in the field as visible, side-by-side, dark, wigglylines on white paper ("seismograms"). At present, the initialreproductions--in a digital format--are on magnetic tape, and finallyare reduced to visible side-by-side traces on paper or film in largecentral computing facilities.

At such centers, sophisticated processing makes possible the distinctionof signals from noise in cases that would have seemed hopeless in theearly days of seismic prospecting. Until 1965, almost all seismicsurveys conducted used an automatic gain control which continuouslyadjusted the gain of amplifiers in the field to account for decreasingamounts of energy from late reflection arrivals. As a result, reflectioncoefficients could not be accurately determined. However, with theadvent of the expander circuit and binary gain amplifiers, gain of theamplifiers can now be controlled and amplitudes recorded precisely; thismakes it possible to conserve not only the special characteristics ofthe reflections, but also their absolute amplitudes.

There was also another problem in the prior art equipment. Computersoften precluded the use of a comparison technique because of their smallword size and tiny core storage. Today, more powerful computers witharray processors and economical floating point capabilities now enablemodern geophysicists to maintain control of the amplitude of allrecorded signals. The "floating point" capability is especiallyeffective in expanding computer work size by a large factor and ineliminating the need for computer automatic gain control.

That is to say, in summary, as a result of the above advances,reflections from many thousands of feet below the earth's surface cannow be confidently detected and followed through sometimes hundreds ofside-by-side traces, the shortening or lengthening of theircorresponding times of arrival being indicative of the shallowing ordeepening of actual sedimentary strata of interest. Still, as a generalrule, all that can be hoped for the seismic reflection method is todetect stratigraphic interfaces and the interfaces as they deviate fromhorizontality of these interfaces, so that subsurface "structures" couldbe defined in which oil or gas might possibly be trapped.

Apropos of the above has been use of ultra-high amplitude anomalies inseismic traces to infer the presence of natural gas in situ. Seismicinterpretators have used so-called "bright-spot" analysis to indicateseveral large gas reservoirs in the world, expecially in the Gulf Coastof the United States. Such analysis is now rather common in the oilindustry, but it is not without its critics. Not only cannot thepersistence of such increased amplitude anomalies be taken asconfirmation of the lateral extent of the gas reservoir, but also theanomaly itself (in some cases) may not represent reflections of adiscontinuity of a gas-bearing medium and its over- or underlyingassociated rock strata. E.g., experience has shown that in certainsituations, similar phenomena occur which can confuse the interpreter.E.g., if the shape of the horizon is such that it focuses the energyback to the surface, it may increase the amplitude of one or more of therecords akin to reflections from gas-saturated strata. Lithology of thehorizon--singly and in combination--can also have a similar effect,producing high-amplitude reflections in the absence of gas within thepore space of the stratum of interest. Examples of the latter:conglomeratic zones, hard streaks of silt or lime and lignite beds.

The present invention improves the ability of the seismologist tocorrectly differentiate high-intensity anomalies ofmultiple-point-coverage seismic traces of gas-bearing strata from thoseof similarly patterned reflections of other types of stratigraphicconfigurations containing no gas accumulations.

OBJECT OF THE INVENTION

An object of the invention is the provision of a novel method ofcorrectly differentiating high-intensity anomalies provided bymultiple-point seismic traces of gas-bearing structures from those of asimilarly patterned intensity associated with strata containing no gasaccumulation.

SUMMARY OF THE INVENTION

In accordance with the present invention, interpretation ofhigh-intensity seismic events from traces obtained from multiple-pointcoverage of a subterranean earth formation using an array of sourcemeans and detectors adjacent to the earth's surface is obtained toindicate gas-bearing strata in a highly surprising and accurate manner.After the field data have been obtained in which the data of commoncenterpoints are associated with more than one source-detector pair, thedata are indexed ("addressed") whereby all recorded traces are indicatedas being a product of respective source-detector pairs of knownhorizontal offset and centerpoint location. Thereafter, high-intensityamplitude anomalies in said traces are correctly associated withgas-bearing strata on a surprisingly accurate selection basis: amplitudeintensity of said anomaly must change--progressively in an increasing ordecreasing manner--as a function of horizontal offset. A furtherrefinement of the method of the present invention may be in order undersome circumstances. A single common-centerpoint trace, or even singlecommon-centerpoint gathers, may have a major drawback in suchcases--poor signal-to-noise ratios. As a result, progressive amplitudechange as a function of offset cannot be resolved. In accordance withthis invention and as a means of signal enhancement, trace summationscan prove beneficial in improving record resolution, say on a basis of astacking "window" having a two-dimensional index for addressing thetraces: X common offset dimension long by Y common centerpoints wide.For example, where 2400% common-centerpoint stacked traces have beenobtained (i.e., 24 traces per gather) by multiple-point-coverage fieldtechniques, each gather can in turn be "de-stacked" to provide originalbut corrected locational traces. Then on the basis of a stacking windowfour (4) common offset dimensions long by five (5) common centerpointswide, several such traces, say 10, can be stacked and the stacked tracedisplayed as a function of offset. Result: progressive change inamplitude intensity as a function of similar intensity changes in offsetcan be more easily observed whereby in situ gas is, more likely thannot, in the pore space of the structure of interest.

DESCRIPTION OF THE DRAWINGS

Further features of the invention will become more apparent uponconsideration of the following detailed description of the inventionwhen taken in connection with the accompanying drawings, wherein:

FIG. 1 is a plan view of a grid of centerpoints produced in the field bythe systematic positioning and energization of an array of seismicsources and detectors whereby a series of locational traces associatedwith individual centerpoints between respective source-detector pairsare ultimately generated;

FIG. 2 is a model of typical reflecting horizons within an earthformation that can be associated with the characteristics of thelocational traces of FIG. 1;

FIGS. 3, 4 and 5 are plots of reflection coefficient as a function ofangle of incidence of seismic waves associated with the reflectinghorizons of FIG. 2 which aid in the determination of the presence of gaswithin an earth formation; FIGS. 6(a), 6(b), 6(c) and 6(d) are plots ofvarious quantities of a mathematical nature, as a function of percentageof gas saturation, illustrating the relationship of Poisson's ratio tothe determination of the presence of gas within an earth formation;

FIGS. 7 and 8 are plots of centerpoints produced by an array of sourcesand detectors wherein a geometrical transformation has occurred tobetter illustrate processes associated with the method of the presentinvention;

FIGS. 9(a) and 9(b) are flow diagrams of processes akin to those shownin FIGS. 7 and 8 for carrying out the method of the present invention,using a programmed digital computing system;

FIGS. 10 and 11 are schematic diagrams of elements within the digitalcomputing system of FIG. 9; and

FIGS. 12-21 are true seismic record sections and portions of sections,illustrating the diagnostic capability of the method of the presentinvention in predicting the presence of gas strata in actual fieldexamples.

PREFERRED EMBODIMENTS OF THE INVENTION

Before discussion of an embodiment of the invention within an actualfield environment, a brief description of the mathematical andtheoretical concepts behind it may prove beneficial and are presentedbelow.

Firstly, it may be of interest to indicate lithology limitationsassociated with the present invention. For example, anomalies associatedwith gas sands over shale cap rock are one example in which the methodof the present invention offers surprising results; another relates togas-saturated limestone over shale. Also of import is the relationshipbetween Poisson's ratio and resulting high-intensity amplitude anomaliesprovided on seismic traces.

While Poisson's ratio (σ) has the general formula ##EQU2## where V_(p)is compressional velocity and V_(s) is shear velocity of the medium, theconcept does have physical significance. For example, consider a slendercylindrical rod of an elastic material and apply a compressional forceto the ends. As the rod changes shape, the length of the rod willdecrease by ΔL, while the radius will increase by ΔR. Poisson's ratio isdefined as the ratio of the relative change in radius (ΔR/R) to therelative change in length (ΔL/L). Hence a compressible material has alow Poisson's ratio, while an incompressible material (as a liquid) hasa high Poisson's ratio.

The equation above also indicates the relationship of the compressionaland shear wave velocities of the material, V_(p) and V_(s) respectively;i.e., that Poisson's ratio may be determined dynamically by measuringthe P-wave and S-wave velocities. Only two of the three variables areindependent, however.

Recent published studies on reflection and transmission seismic wavesuseful in geophysical applications include:

(1) Koefoed, O., 1955, for "On the Effect of Poisson's Ratios of RockStrata in the Reflection Coefficients of Plane Waves", GeophysicalProspecting, Vol. 3, No. 4.

(2) Koefoed, O., 1962, for "Reflection and Transmission Coefficients forPlane Longitudinal Incident Waves", Geophysical Prospecting, Vol. 10,No. 3.

(3) Muskat, M. and Meres, M. W., 1940, for "Reflection and TransmissionCoefficients for Plane Waves in Elastic Media", Geophysics, Vol. 5, No.2.

(4) Tooley, R. D., Spencer, T. W. and Sagoci H. F., for "Reflection andTransmission of Plane Compressional Waves", Geophysics, Vol. 30, No. 4(1965).

(5) Costain, J. K., Cook, K. L. and Algermisshi, S. T., for "Amplitude,Energy and Phase Angles of Plane SP Waves and Their Application to EarthCrustal Studies", Bull. Seis. Soc. Am., Vol. 53, p. 1639 et. seq.

All of the above have focused on the complex modeling of variation inreflection and transmission coefficients as a function of angle ofincidence.

The problem is complicated, however, E.g., isotropic media with layerindex of the strata, i=1 for the incident medium and i=2 for theunderlying medium, have been modeled using equations for P-wavereflection coefficient A_(pr) and for P-wave transmission displacementamplitude coefficient A_(pt). In such equations, the value of Poisson'sratio (σ) is required, since both the P-wave and S-wave velocities areutilized. For each of the media, i.e., the incident or underlyingmedium, three independent variables exist: P-wave velocity, σ and bulkdensity, or a total of six variables for both media. But for a singleinterface, only four independent variables were required: P-wavevelocity ratio, the density ratio, Poisson's ratio in the incidentmedium and Poisson's ratio in the underlying medium.

But to provide for the many combinations of possible variations, theabove-listed studies have either:

(a) generated many (literally thousands) plots of a mathematical naturefor various parameters, values in which there was little relationshipwith true geophysical applications, since the latter were hopelesslyobscured and unappreciated; or

(b) made simplistic assumptions that, although using actualcalculations, nevertheless did not express the true nature oftransmission and reflection coefficients, in particular lithologicalsituations associated with the accumulation of gaseous hydrocarbonswithin an acutal earth formation.

In summary, while reference (2) concluded that change in Poisson's ratioin the two bounding media can cause change in the reflection coefficientas a function of angle of incidence, (2) did not relate that occurrenceto lithology associated with the accumulation of gaseous hydrocarbons inthe surprising manner of the present invention.

The present invention teaches that gas-containing strata have lowPoisson's ratios and that the contrast with the overburden rock as afunction of horizontal offset produces a surprising result: suchcontrast provides for a significant--and progressive--change in P-wavereflection coefficient at the interface of interest as a function ofangle of incidence of the incident wave. Thus, differentiating betweenhigh-intensity amplitude anomalies of nongas and gas media is simplifiedby relating progressive change in amplitude intensity as a function ofoffset between source-detector pairs, i.e., angle of incidence beingdirectly related to offset.

Also, the behavior of P-wave travel as a function of lithology andhorizontal offset between a respective source-detector pair associatedwith a given locational trace provide the following amplitude responsesignatures of interest:

(1) where the gas-containing media are gas sands underlying shale, suchas found in the Gulf Coast, amplitude responses increase with offset;

(2) where the gas-containing media include limestone underlying shale,such as found in the North Sea, the amplitude anomalies of the interfacedecrease with offset.

Now in more detail, attention should be directed to the Figures,particularly FIG. 1. Note that, inter alia, FIG. 1 illustrates in somedetail how the terms of interest in this application are derived: e.g.,the term "centerpoint" is a geographical location located midway betweena series of sources S₁,S₂ . . . S_(n) of a geophysical field system 9and a set of detectors D₁,D₂ . . . D_(m) at a datum horizon near theearth's surface. The centerpoints are designated C₁,C₂ . . . C_(p) inthe Figure, and are associated with a trace derived by placement of asource at that centerpoint location followed immediately by relocating adetector thereat.

I.e., if the sources S₁ . . . S_(n) are excited in sequence at thesource locations indicated, traces received at the different detectorlocations shown can be related to common centerpoints therebetween. Ifsuch traces are summed, a gather or group of traces is formed. I.e., ifthe reflecting interface is a flat horizon, the depth point wherereflection occurs will define a vertical line which passes through thecenterpoint of interest. Applying static and dynamic corrections to thefield traces is equivalent (under the above facts) to placing theindividual sources S₁,S₂ . . . S_(n) at the centerpoint in sequencefollowed by replacement with the detectors D₁ . . . D_(m) of interest atthe same locations. If the traces associated with a common centerpointare summed, a series of enhanced traces, sometimes called CDPS (CommonDepth Point Stack) traces, is provided.

FIG. 2 illustrates reflection phenomena of a three-layer model typicalof a young, shallow geologic section 10, such as found in the GulfCoast, illustrating how reflection phenomena associated with the tracesassociated with the field system 9 of FIG. 1 can be related to thepresence of gas.

Section 10 includes a gas sand 11 embedded in a shale stratum 12. Assumea Poisson's ratio of 0.1 for the gas sand and of 0.4 for the shale, a20% velocity reduction at interface 13, say from 10,000'/sec to8000'/sec, and a 10% density reduction from 2.40 g/cc to 2.16 g/cc.

The actual P-wave reflection coefficient Apr can be related to section10 by Equation (1) below; also, P-wave transmission displacementamplitude coefficient Apt can similarly be related in accordance withEquation (2) below. ##EQU3##

Equations (1) and (2) are, of course, the two basic equations of wavetravel in an earth formation and are for isotropic media with the layerindex being i=1 for the incident medium and i=2 for the underlyingmedium. Equations (3) through (17) simply define intermediate variables.

As an example of calculations associated therewith, if θ=0° (normalincidence), the P-wave reflection coefficient Apr is equal to about-0.16 and +0.16, respectively.

FIG. 3 illustrates change in reflection coefficient as a function ofangle of incidence θ for the three-layer model of FIG. 2.

Note that solid lines 20, 21 illustrate the effects of reflection (andtransmission, by omission) on the top and base of the gas sand. In line20, at θ=0°, note that the A_(pr) equals -0.16; while at θ=40°, theA_(pr) is about -0.28. That is, rather a surprisingly large change inthe reflection coefficient as a function of angle of incidence occurs,with the greatest change occurring between θ=20° and θ=40°.

For the bottom layer, line 21 changes at about the same rate, but inopposite sign. I.e., at θ=0°, A_(pr) is about +0.16 and at θ=40°, A_(pr)is about +0.26. Again, the greatest change in A_(pr) occurs betweenθ=20° and θ=40°. As a result, the amplitude of the seismic wavereflected from this model would increase about 70% over the angle ofincidence range shown, i.e., over the incremental 40 degrees shown.

While angles of incidence equal to 40° may seem a little large forreflection profiling (heretofore, most data arriving beyond 30° beingthought useless and muted out), experience has now nevertheless shownthat reflection data can and do arrive at reflection angles greater than30°. Hence, the angles of incidence must be determined, and one of themore important techniques, the straight-ray approach to estimate suchangles of incidence (using depth-to-reflector and shot-to-detectorand-shot-to-group offset), is as set forth below:

    θ.sub.1 =arc tan (X/2Z)                              (18)

where θ₁ is the angle of incidence; X is the shot-to-detector orshot-to-group offset and Z is the reflector depth. Velocity changes withdepth can likewise be accommodated by assuming section velocity changeis of the form V₁ =V_(o) +KZ where K is a constant so that all ray pathsare arcs of circles having centers V_(o) /K above the reference plane ofinterest, say the earth's surface. Thus, the approach should be inaccordance with ##EQU4##

Having now established a firm mathematical and theoretical basis for theprocess of the present invention, perhaps a description of how ageological section containing no gas therein would affect impedancecontrast is in order. FIG. 4 illustrates the changes in reflectioncoefficient as a function of angle of incidence θ in the manner of FIG.3, but in which the gas sand 10 of FIG. 2 contains no gas, simulating,e.g., a low-velocity, brine-saturated, young sandstone embedded inshale.

The solid lines 22, 23, representing reflection coefficients, are seento be about horizontal between θ=0° and 40°, slightly decreasing inmagnitude as the angle of incidence increases, i.e., as θ approaches40°. In the above example, it should be noted that the Poisson's ratioof the sandstone was assumed to be 0.4.

FIG. 5 illustrates yet another plot associated with a three-layer modelakin to that shown in FIG. 2, but in which the sandstone contains gasbut is buried deep below the earth's surface. The values for thethree-layer model of FIG. 2 are again used except that the velocitychange from shale to sand is only 10%, or from 10,000'/sec to 9000'/sec.As shown, curves 25, 26 are even more significant: both curves are seento increase in magnitude from over the 40° of change in the angle ofincidence. However, field results have not verified these results, sincePoisson's ratio in such gas sands may be strongly affected by depth, andnot be as low as is now surprisingly taught by the present invention.

FIGS. 6(a), 6(b), 6(c) and 6(d) offer a possible explanation for lowPoisson's ratio in gas-containing strata in general and in gas sands inparticular. In the Figures, various quantities are plotted as a functionof percentage of gas saturation. In FIG. 6(a), P-wave velocity is soplotted; in FIG. 6(b), S-wave velocity is depicted; in FIG. 6(c), theratio of Vp/Vs is the value of interest; and in FIG. 6(d), Poisson'sratio is shown as a function of percent gas saturation.

Note that FIGS. 6(a) and 6(b) are for sandstones buried at 6000 feetwith 35% porosity. FIGS. 6(c) and 6(d) result from FIGS. 6(a) and 6(b)using appropriate equations. But in FIG. 6(d), Poisson's ratio dropsfrom about 0.3 to 0.1 from 0% to 10% gas saturation; on the other hand,the same ratio changes very little from 10% to 100% gas saturation(average value is about 0.09).

Hence, from the above mathematical and theoretical concepts, displays ofreflection data can now be used to indicate change in reflectioncoefficient as a function of angle of incidence to indicate the presenceof gaseous hydrocarbons. Such data are also now conveniently available,say using today's conventional field-gathering techniques involvingmultiple-area coverage, since the former can be derived from and iscompatible with one of today's conventional forms of recorded reflectionseismic data: common-depth-point (CDP) gathers. And, progressive changesin reflection amplitude vs. shot-to-detector (group) offset can form thebasis of such a determination, since offset of any particularsource-detector pair is directly related to the angle of incidence inaccordance with Equation (19), supra.

But corrected locational trace data, before stacking in accordance withCDPS techniques, often have poor signal-to-noise ratios. Thus, changesin amplitude vs. offset may be difficult to observe in such data.

FIG. 7 is a diagram which illustrates a data "addressing" techniquewhich improves amplitude versus offset resolution in such situations; inthe Figure, the traces were generated using an end-shooting array of 48detectors with source and detectors advancing one detector interval pershot point. Result: a 24-fold CDP-stacked record section was generated.Note further: each centerpoint is associated with 24 separate traces ofvarying offset.

In order to geometrically associate each generated locational trace withits common centerpoint, address guidance, as provided by FIG. 7, isimportant. To understand the nature of FIG. 7, assume that the sourcesS₁,S₂ . . . S_(n) are sequentially located at shotpoints SP₁,SP₂ . . .SP_(n) at the top of the Figure. Assume also that the detectors areplaced in line with the sources, i.e., along the same line of survey Aat the detector locations D₁,D₂ . . . D_(m). After each source isactivated, reflections are received at the detectors, at the locationsshown. Then by the "rollalong" technique, the source and detectorspreads can be moved in the direction B of survey line A and the processrepeated to provide a series of traces. The latter are associated withcenterpoints midway between the respective detector-source pairs. In theFigure, assume source S₁ has been located at shotpoint SP₁ and excited.Midway between SP₁ and each of the detectors, at D₁, D₂ . . . D_(m), isa series of centerpoints C₁, C₂ . . . C_(n). The latter are eachassociated with a trace. In this regard and for a further description ofsuch techniques, see U.S. Pat. No. 3,597,727 for "Method of AttenuatingMultiple Seismic Signals in the Determination of Inline and Cross-DipsEmploying Cross-Steered Seismic Data", Judson et al, issued Aug. 3,1971, and assigned to the assignee of the present application. Withappropriate static and dynamic corrections, the data can be related tothe common centerpoints midway between individual source points anddetectors, as discussed in the above-noted reference.

But by such a field technique, data provided generate 24 separate tracesassociated with the same centerpoint C₁ . . . C_(n). In order to index("address") these traces as a function of several factors includinghorizontal offset and centerpoint location, a stacking chart 44 as shownin FIG. 8 has been developed.

Chart 44 is a diagram in which a trace is located along a plurality ofoblique common profile lines PL₁,PL₂ . . . , between a series of commonoffset and centerpoint locations at 90 degrees to each other. For bestillustration, focus on a single shotpoint, say SP₁, and on a singledetector spread having detectors D₁,D₂ . . . D_(m) of FIG. 8 alongsurvey line A. Assume a source is located at shotpoint SP₁ and activatedthereafter. The detector spread and source are "rolled" forward alongsurvey line A in the direction B, being advanced one station peractivation. Then after detection has occurred, and if the resultingcenterpoint pattern is rotated 45° about angle 46 to profile line PL₁and projected below the spread as in FIG. 8 as a function of commonoffset values and centerpoint positions, the chart 44 of FIG. 8 results.Of course, each centerpoint has an amplitude vs. time trace associatedtherewith, and for didactic purposes that trace can be said to projectalong a line normal to the plane of the Figure.

It should be emphasized that the centerpoints provided in FIGS. 7 and 8are geographically located along the line of survey A in line with thesource points SP₁,SP₂ . . . . As the locational traces are generated,the chart 44 aids in keeping a "tag" on each resulting trace. As thedetector spread and sources are rolled forward one station and thetechnique repeated, another series of traces is generated associatedwith centerpoints on new profile line PL₁. That is, although thecenterpoints are geographically still associated within positions alongthe survey line A of FIG. 7, by rotation along the angle 46, the newcenterpoint pattern C₁ ',C₂ ' . . . C_(n) ' can be horizontally andvertically aligned with centerpoints previously generated. I.e., atcommon offset values (in horizontal alignment) certain centerpoints arealigned, viz, centerpoint C₁ aligns with C₁ ' as shown; further C₂ isaligned with C₂ ', etc. Also, there are traces that have commoncenterpoints. I.e., at common centerpoints (in vertical alignment)centerpoint C₂ aligns with centerpoint C₁ ', and centerpoints C₃,C₂ 'and C₁ " are similarly aligned. Thus, via chart 44, each traceassociated with a centerpoint can be easily "addressed" as to:

(i) its actual geographical location (i.e., along phantom lines normalto diagonal profile lines PL₁,PL₂ . . . along common location linesLL₁,LL₂ . . . ), so that its actual field location is likewise easilyknown;

(ii) its association with other traces along common horizontal offsetlines COL₁,COL₂ . . . COL_(x) ; and

(iii) its association with still other traces along common verticalcenterpoint location lines CPL₁,CPL₂ . . . .

Also, "addressing" the traces by (ii) and (iii) allows such traces to beeasily combined (summed) by calling out "windows" within the chart inwhich any traces within the window can be summed. E.g., it has beenfound convenient to establish a standard window "width" equal to anincreased group centerpoint line (ΔCPL) value of say 5, and a window"height" equal to an incremental common group offset line (ΔCOL) valueof say 4; hence by indexing the intersecting window intervals on asequential basis, summation of traces therein can occur. The results aresummed traces which are outputted to a display on a side-by-side basis,say as a function of amplitude intensity as a function of increasing ordecreasing offset between respective source-detector pairs. Actualoffset values are not required, since relative values are usuallysufficient for most diagnostic purposes.

In carrying out the above summation process on a highspeed basis, afully programmed digital computer can be useful. But electromechanicalsystems well known in the art can also be used. In either case, thefield traces must first undergo static and dynamic correction before thetraces can be displayed as a function of offset to determine theirpotential as a gas reservoir. Such correction techniques are well knownin the art--see, e.g., U.S. Pat. No. 2,838,743, of O. A. Fredriksson etal, for "Normal Moveout Correction with Common Drive for RecordingMedium and Recorder and/or Reproducing Means", assigned to the assigneeof the present application, in which a mechanical device and method aredepicted. Modern processing today uses properly programmed digitalcomputers for that task in which the data words are indexed as afunction of, inter alia, amplitude, time, datum height, geographicallocation, group offset, velocity, and are manipulated to correct for theangular and horizontal offset; in this latter environment, see U.S. Pat.No. 3,731,269, Judson et al, issued May 1, 1973, for "Static Correctionsfor Seismic Traces by Cross-Correlation Method", a computer-implementedprogram of the above type also assigned to the assignee of the presentinvention. Electromechanical sorting and stacking equipment is also wellknown in the art and is of the oldest ways of cancelling noise. See, forexample, the following patents assigned to the assignee of the presentinvention which contain sorting and stacking techniques, including beamsteering techniques:

    ______________________________________                                                        In-                                                           Patent Issued   ventor  Title                                                 ______________________________________                                        3,597,727                                                                             12/30/68                                                                              Judson  Method of Attenuating Multiple                                        et al   Seismic Signals in the Deter-                                                 mination of Inline and Cross                                                  Dips Employing Cross-Steered                                                  Seismic Data                                          3,806,863                                                                            4/23/74  Tilley  Method of Collecting Seismic                                          et al   Data of Strata Underlying                                                     Bodies of Water                                       3,638,178                                                                            1/25/72  Ste-    Method for Processing Three-                                          phen-   Dimensional Seismic Data to                                           son     Select and Plot Said Data on                                                  a Two-Dimensional Display                                                     Surface                                               3,346,840                                                                             10/10/67                                                                              Lara    Double Sonogramming for                                                       Seismic Record Improvement                            3,766,519                                                                             10/16/73                                                                              Step-   Method for Processing Surface                                         phen-   Detected Seismic Data to                                              son     Plotted Representations of Sub-                                               surface Directional Seismic Data                      3,784,967                                                                            1/8/74   Graul   Seismic Record Processing Method                      3,149,302                                                                            9/15/74  Klein   Information Selection Programmer                                      et al   Employing Relative Amplitude,                                                 Absolute Amplitude and Time                                                   Coherence                                             3,149,303                                                                            9/15/64  Klein   Seismic Cross-Section Plotter                                         et al                                                         ______________________________________                                    

FIGS. 9(a) and 9(b) are flow diagrams illustrative of acomputer-dominated process in which the functions required by the methodof the present invention can be easily ascertained. Preliminary to thesteps shown in FIG. 9(a), assume that a section of seismic data has beenanalyzed for "bright spots"; such events are known by geographicallocation and/or a time/depth basis; and the traces have been dynamicallyand statically corrected.

The steps of FIG. 9(a) include generating addresses for the data thatinclude a common offset address in the manner of FIG. 8, commoncenterpoint address and an actual geographical location address in themanner of FIG. 8. Finally, the corrected traces are classified wherebythe amplitude event of interest is displayed as a function of changinghorizontal offset valves. If the event progressively changes as afunction of offset, there is a high likelihood that the event isindicative of strata containing gaseous hydrocarbons.

After the addresses have been generated, trace amplitude summation canalso occur as suggested in FIG. 9(b) on a predetermined selection basis:adding traces within a selected "window", say window 49 bounded bycommon group centerpoint lines (ΔCPL) and common group offset lines(ΔCOL) (see FIG. 8), usually the "width" (ΔCPL) of window 49 is heldconstant, and the window "height" (ΔCOL) is incremented, frame-by-frame,to change common offset values on a progressive basis, say from nearoffset values to far offset values. E.g., in FIG. 8, holding the window"width" constant and beginning at the lower boundary of the chart 44,the window "height" (ΔCOL) can be incremented until the upper plotboundary is reached. Equipment-wise, the addresses of the boundary linesin the line and column directions are compared--within each windowframe. When the comparison is a match, the address register isincremented, and the process repeated for the next window frame.

After the summed traces are tagged on the basis of changing commonoffset, say near through far offset, summed traces can be displayed. Ifthe amplitudes exhibit the required characteristics, as previouslystated, then a determination that the reflections are from (or not from)a gas structure can be made.

Of course, if the edge of a gas field is to be determined, theabove-mentioned process would be sequentially elongated toward a side ofthe chart 44 of FIG. 8 in a direction along the survey line. I.e., tosay, after the window "height" has been incremented to its far offsetvalue and the results displayed, the window "width" is incremented acommon group centerpoint interval (ΔCPL) and the process repeated andthe results displayed.

FIG. 10 illustrates particular elements of a computing system forcarrying out the steps of FIGS. 9(a) and 9(b). While many computingsystems are available to carry out the process of the invention, perhapsto best illustate operations at the lowest cost per instruction, amicrocomputing system 50 is didactically best and is presented in detailbelow. The system 50 of FIG. 10 can be implemented on hardware providedby many different manufacturers, and for this purpose, elements providedby Intel Corporation, Santa Clara, California, may be preferred.

Such a system 50 can include a CPU 51 controlled by a control unit 52.Two memory units 53 and 54 connect to the CPU 51 through BUS 55. Programmemory unit 53 stores instructions for directing the activities of theCPU 51 while data memory unit 54 contains data (as data words) relatedto the seismic data provided by the field acquisition system. Since theseismic traces contain large amounts of bit data, an auxiliary memoryunit 56 can be provided. The CPU 51 can rapidly access data storedthrough addressing the particular input port, say at 57a in the Figure.Additional input ports can also be provided to receive additionalinformation as required from usual external equipment well known in theart, e.g., floppy disks, paper-tape readers, etc., including suchequipment interfaced through input interface port 57b tied to a keyboardunit 58 for such devices. Using clock inputs, control circuity 52maintains the proper sequence of events required for any processingtask. After an instruction is fetched and decoded, the control circuitryissues the appropriate signals (to units both internal and external) forinitiating the proper processing action. Often the control circuitrywill be capable of responding to external signals, such as an interruptor wait request. An interrupt request will cause the control circuitry52 to temporarily interrupt main program execution, jump to a specialroutine to service the interrupting device, then automatically return tothe main program. A wait request is often issued by memory units 53 or54 or an I/O element that operates slower than the CPU.

For outputting information, the system 50 can include a printer unit 59whereby the amplitude of the summed traces as a function of time isprintable. Of more use as an output unit, however, is disk unit 60,which can temporarily store the data. Thereafter, an off-line digitalplotter capable of generating a side-by-side display is used inconjunction with the data on the disk unit 60. Such plotters areavailable in the art, and one proprietary model that I am familiar withuses a computer-controlled CRT for optically merging onto photographicpaper, as a display mechanism, the seismic data. Briefly, in such aplotter the seismic data, after summation, are converted to CRTdeflection signals; the resulting beam is drawn on the face of the CRTand the optically merged record of the event indicated, say viaphotographic film. After a predetermined number of side-by-side lineshave been drawn, the film is processed in a photography laboratory andhard copies returned to the interpreters for their review.

FIG. 11 illustrates CPU 51 and control unit 52 in more detail.

As shown, the CPU 51 includes an array of registers generally indicatedat 62 tied to an ALU 63 through an internal data bus 64 under control ofcontrol unit 52. The registers 62 are temporary storage areas. Programcounter 65 and instruction register 66 have dedicated uses; the otherregisters, such as accumulator 67, have more general uses.

The accumulator 67 usually stores one of the seismic operands to bemanipulated by the ALU 63. E.g., in the summation of traces, theinstruction may direct the ALU 63 to not only add in sequence thecontents of the temporary registers containing predetermined traceamplitudes together with an amplitude value in the accumulator, but alsostore the result in the accumulator itself. Hence, the accumulator 67operates as both a source (operand) and a destination (result) register.The additional registers of the array 62 are useful in manipulation ofseismic data, since they eliminate the need to shuffle results back andforth between the external memory units of FIG. 10 and accumulator 67.In practice most ALU's also provide other built-in functions, includinghardware subtraction, boolean logic operations, and shift capabilities.The ALU 63 also can utilize flag bits generated by FF unit 73 whichspecify certain conditions that arise in the course of arithmetical andlogical manipulations. Flags typically include carry, zero, sign, andparity. It is possible to program jumps which are conditionallydependent on the status of one or more flags. Thus, for example, theprogram may be designed to jump to a special routine if the carry bit isset following an addition instruction.

Instructions making up the program for operations involving seismic dataare stored in the program memory unit 53 of the CPU 51 of FIG. 11. Theprogram is operated upon in a sequential manner except when instructionsin the memory units 53, 54 call for special commands such as "jump" (or"call") instructions. While the program associated with the presentinvention is a relatively straightforward one, hence avoiding most"jump" and "call" instructions, "call" instructions for subroutines arecommon in the processing of seismic data and could be utilized, ifdesired. In "call" instructions, the CPU 51 has a special way ofhandling subroutines in order to insure an orderly return to the mainprogram. When the processor receives a call instruction, it incrementsthe program counter 65 and notes the counter's contents in a reversedmemory area of the memory unit known as the "stack".

CPU's have different ways of maintaining stack contents. Some havefacilities for the storage of return addresses built into the CPUitself. Other CPU's use a reserved area of external memory as the stackand simply maintain a "pointer" register, such as pointer register 70,FIG. 11, which contains the address of the most recent stack entry. Thestack thus saves the address of the instruction to be executed after thesubroutine is completed. Then the CPU 51 loads the address specified inthe call into its program counter 65. The next instruction fetched willtherefore be the first step of the subroutine. The last instruction inany subroutine is a "return". Such an instruction need specify noaddress.

Having now briefly described the operations of the CPU 51, Table I ispresented below containing a full instruction set for its operations.

                                      TABLE I                                     __________________________________________________________________________    Summary Of Processor Instructions By Alphabetical Order                                             Instruction Code.sup.1                                                                        Clock.sup.2                             Mnemonic                                                                             Description    D.sub.7                                                                         D.sub.6                                                                         D.sub.5                                                                         D.sub.4                                                                         D.sub.3                                                                         D.sub.2                                                                         D.sub.1                                                                         D.sub.0                                                                         Cycles                                  __________________________________________________________________________    ACI    Add immediate to A with                                                       carry          1 1 0 0 1 1 1 0 7                                       ADC M  Add memory to A with carry                                                                   1 0 0 0 1 1 1 0 7                                       ADC r  Add register to A with                                                        carry          1 0 0 0 1 S S S 4                                       ADD M  Add memory to A                                                                              1 0 0 0 0 1 0 1 7                                       ADD r  Add register to A                                                                            1 0 0 0 0 S S S 4                                       ADI    Add immediate to A                                                                           1 1 0 0 0 1 1 0 7                                       ANA M  And memory with A                                                                            1 0 1 0 0 1 1 0 7                                       ANA r  And register with A                                                                          1 0 1 0 0 S S S 4                                       ANI    And immediate with A                                                                         1 1 1 0 0 1 1 0 7                                       CALL   Call unconditional                                                                           1 1 0 0 1 1 0 1 17                                      CC     Call on carry  1 1 0 1 1 1 0 0 11/17                                   CM     Call on minus  1 1 1 1 1 1 0 0 11/17                                   CMA    Compliment A   0 0 1 0 1 1 1 1 4                                       CMC    Compliment carry                                                                             0 0 1 1 1 1 1 1 4                                       CMP M  Compare memory with A                                                                        1 0 1 1 1 1 1 0 7                                       CMP r  Compare register with A                                                                      1 0 1 1 1 S S S 4                                       CNC    Call on no carry                                                                             1 1 0 1 0 1 0 0 11/17                                   CNZ    Call on no zero                                                                              1 1 0 0 0 1 0 0 11/17                                   CP     Call on positive                                                                             1 1 1 1 0 1 0 0 11/17                                   CPE    Call on parity even                                                                          1 1 1 0 1 1 0 0 11/17                                   CPI    Compare immediate with A                                                                     1 1 1 1 1 1 1 0 7                                       CPO    Call on parity odd                                                                           1 1 1 0 0 1 0 0 11/17                                   CZ     Call on zero   1 1 0 0 1 1 0 0 11/17                                   DAA    Decimal adjust A                                                                             0 0 1 0 0 1 1 1 4                                       DAD B  Add B & C to H & L                                                                           0 0 0 0 1 0 0 1 10                                      DAD D  Add D & E to H & L                                                                           0 0 0 1 1 0 0 1 10                                      DAD H  Add H & L to H & L                                                                           0 0 1 0 1 0 0 1 10                                      DAD SP Add stack pointer to H & L                                                                   0 0 1 1 1 0 0 1 10                                      DCR M  Decrement memory                                                                             0 0 1 1 0 1 0 1 10                                      DCR r  Decrement register                                                                           0 0 D D D 1 0 1 5                                       DCX B  Decrement B & C                                                                              0 0 0 0 1 0 1 1 5                                       DCX D  Decrement D & E                                                                              0 0 0 1 1 0 1 1 5                                       DCX H  Decrement H & L                                                                              0 0 1 0 1 0 1 1 5                                       DCX SP Decrement stack pointer                                                                      0 0 1 1 1 0 1 1 5                                       DI     Disable interrupt                                                                            1 1 1 1 0 0 1 1 4                                       EI     Enable interrupts                                                                            1 1 1 1 1 0 1 1 4                                       HLT    Halt           0 1 1 1 0 1 1 0 7                                       IN     Input          1 1 0 1 1 0 1 1 10                                      INR M  Increment memory                                                                             0 0 1 1 0 1 0 0 10                                      INR r  Increment register                                                                           0 0 D D D 1 0 0 5                                       INX B  Increment B & C registers                                                                    0 0 0 0 0 0 1 1 5                                       INX D  Increment D & E registers                                                                    0 0 0 1 0 0 1 1 5                                       INX H  Increment H & L registers                                                                    0 0 1 0 0 0 1 1 5                                       INX SP Increment stack pointer                                                                      0 0 1 1 0 0 1 1 5                                       JC     Jump on carry  1 1 0 1 1 0 1 0 10                                      JM     Jump on minus  1 1 1 1 1 0 1 0 10                                      JMP    Jump unconditional                                                                           1 1 0 0 0 0 1 1 10                                      JNC    Jump on no carry                                                                             1 1 0 1 0 0 1 0 10                                      JNZ    Jump on no zero                                                                              1 1 0 0 0 0 1 0 10                                      JP     Jump on positive                                                                             1 1 1 1 0 0 1 0 10                                      JPE    Jump on parity even                                                                          1 1 1 0 1 0 1 0 10                                      JPO    Jump on parity odd                                                                           1 1 1 0 0 0 1 0 10                                      JZ     Jump on zero   1 1 0 0 1 0 1 0 10                                      LDA    Load A direct  0 0 1 1 1 0 1 0 13                                      LDAX B Load A indirect                                                                              0 0 0 0 1 0 1 0 7                                       LDAX D Load A indirect                                                                              0 0 0 1 1 0 1 0 7                                       LHLD   Load H & L direct                                                                            0 0 1 0 1 0 1 0 16                                      LXI B  Load immediate register                                                       Pair B & C     0 0 0 0 0 0 0 1 10                                      LXI D  Load immediate register                                                       Pair D & E     0 0 0 1 0 0 0 1 10                                      LXI H  Load immediate register                                                       Pair H & L     0 0 1 0 0 0 0 1 10                                      LXI SP Load immediate stack                                                          pointer        0 0 1 1 0 0 0 1 10                                      MVI M  Move immediate memory                                                                        0 0 1 1 0 1 1 0 10                                      MVI r  Move immediate register                                                                      0 0 D D D 1 1 0 7                                       MOV M,r                                                                              Move register to memory                                                                      0 1 1 1 0 S S S 7                                       MOV r,M                                                                              Move memory to register                                                                      0 1 D D D 1 1 0 7                                       MOV r.sub.1,r.sub.2                                                                  Move register to register                                                                    0 1 D D D S S S 5                                       NOP    No operation   0 0 0 0 0 0 0 0 4                                       ORA M  Or memory with A                                                                             1 0 1 1 0 1 1 0 7                                       ORA r  Or register with A                                                                           1 0 1 1 0 S S S 4                                       ORI    Or immediate with A                                                                          1 1 1 1 0 1 1 0 7                                       OUT    Output         1 1 0 1 0 0 1 1 10                                      PCHL   H & L to program counter                                                                     1 1 1 0 1 0 0 1 5                                       POP B  Pop register pair B & C                                                       off stack      1 1 0 0 0 0 0 1 10                                      POP D  Pop register pair D & E                                                       off stack      1 1 0 1 0 0 0 1 10                                      POP H  Pop register pair H & L                                                       off stack      1 1 1 0 0 0 0 1 10                                      POP PSW                                                                              Pop A and Flags off stack                                                                    1 1 1 1 0 0 0 1 10                                      PUSH B Push register Pair B & C                                                      on stack       1 1 0 0 0 1 0 1 11                                      PUSH D Push register Pair D & E                                                      on stack       1 1 0 1 0 1 0 1 11                                      PUSH H Push register Pair H & L                                                      on stack       1 1 1 0 0 1 0 1 11                                      PUSH PSW                                                                             Push A and Flags on stack                                                                    1 1 1 1 0 1 0 1 11                                      RAL    Rotate A left through                                                         carry          0 0 0 1 0 1 1 1 4                                       RAR    Rotate A right through                                                        carry          0 0 0 1 1 1 1 1 4                                       RC     Return on carry                                                                              1 1 0 1 1 0 0 0 5/11                                    RET    Return         1 1 0 0 1 0 0 1 10                                      RLC    Rotate A left  0 0 0 0 0 1 1 1 4                                       RM     Return on minus                                                                              1 1 1 1 1 0 0 0 5/11                                    RNC    Return on no carry                                                                           1 1 0 1 0 0 0 0 5/11                                    RNZ    Return on no zero                                                                            1 1 0 0 0 0 0 0 5/11                                    RP     Return on positive                                                                           1 1 1 1 0 0 0 0 5/11                                    RPE    Return on parity even                                                                        1 1 1 0 1 0 0 0 5/11                                    RPO    Return on parity odd                                                                         1 1 1 0 0 0 0 0 5/11                                    RRC    Rotate A right 0 0 0 0 1 1 1 1 4                                       RST    Restart        1 1 A A A 1 1 1 11                                      RZ     Return on zero 1 1 0 0 1 0 0 0 5/11                                    SBB M  Subtract memory from A                                                        with borrow    1 0 0 1 1 1 1 0 7                                       SBB r  Subtract register from A                                                      with borrow    1 0 0 1 1 S S S 4                                       SBI    Subtract immediate from A                                                     with borrow    1 1 0 1 1 1 1 0 7                                       SHLD   Store H & L direct                                                                           0 0 1 0 0 0 1 0 16                                      SPHL   H & L to stack pointer                                                                       1 1 1 1 1 0 0 1 5                                       STA    Store A direct 0 0 1 1 0 0 1 0 13                                      STAX B Store A indirect                                                                             0 0 0 0 0 0 1 0 7                                       STAX D Store A indirect                                                                             0 0 0 1 0 0 1 0 7                                       STC    Set carry      0 0 1 1 0 1 1 1 4                                       SUB M  Subtract memory from A                                                                       1 0 0 1 0 1 1 0 7                                       SUB r  Subtract register from A                                                                     1 0 0 1 0 S S S 4                                       SUI    Subtract immediate from A                                                                    1 1 0 1 0 1 1 0 7                                       XCHG   Exchange D & E, H & L                                                         Registers      1 1 1 0 1 0 1 1 4                                       XRA M  Exclusive Or memory                                                           with A         1 0 1 0 1 1 1 0 7                                       XRA r  Exclusive Or register                                                         with A         1 0 1 0 1 S S S 4                                       XRI    Exclusive Or immediate                                                        with A         1 1 1 0 1 1 1 0 7                                       XTHL   Exchange top of stack,                                                        H & L          1 1 1 0 0 0 1 1 18                                      __________________________________________________________________________     .sup.1 DDD or SSS000B-001C-010D-011E-100H-101L-110 Memory111A.                .sup.2 Two possible cycle times (5/11) indicate instruction cycles            dependent on condition flags.                                            

EXAMPLES

Diagnostic capability provided by the method of the present invention isbetter illustrated in the Examples set forth below.

EXAMPLE I

Seismic data were obtained over a gas field near Sacramento, California.These data, in CDP-stacked form, are shown in FIG. 12. The field,discovered in 1972, consists of a 100-foot sand which is almost fullygas-saturated. The discovery well is located at about SP-86 of FIG. 12,with the currently developed portion of the field extending from aboutSP-75 to SP-115. Gas occurs at a depth of about 7000 feet, whichcorresponds to a time of about 1.75 seconds on the plot.

Common-depth-point gathers from 3 locations, A, B and C of FIG. 12, areshown in FIGS. 13(a), 13(b), and 13(c). Both single-fold and 10-foldsummed gathers are shown for locations A and B, while only the summedgather is shown for location C. Shot-to-group offset for all gathersincreases to the left, as indicated with the minimum and maximum traceoffset distances annotated. These distances change on the summed gathersbecause the summing is done over 4 offsets.

Note the strong amplitude increase with increasing offset at locations Aand B. The 10-fold summing obviously improves signal-to-noise ratios andan amplitude increase by a factor of about three is indicated from nearto far offset. Gathers at location C, however, show no indication ofamplitude increase with offset, and in fact show a decrease. Thispossibly indicates an absence of gas in the vicinity of location C. Thispossibility is also supported by the presence of a gas-water contact ina well structurally projected at about SP-120.

EXAMPLE II

Seismic data were obtained in the Fallon Basin of Nevada and aredepicted in CDP-gathered format in FIG. 14. A well was drilled at SP-127in FIG. 4. A seismic amplitude anomaly is indicated at location A atabout 1.6 seconds. Upon drilling, the amplitude anomaly was found tooriginate from two basaltic layers, 100 feet and 60 feet in thickness.As its structural position indicates, this well was a stratigraphic testin an undrilled basin.

The common-depth-point gathers at the well location are shown in FIG.15. Here, there is a strong indication of reflection amplitude decreasewith increasing offset. This finding is consistent with the absence ofgas in the geologic section and the expected Poisson's ratios forsediments and basalt.

EXAMPLE III

Seismic data obtained from an area in the Sacramento Valley, California,are depicted in FIG. 16. A well was drilled at SP-61. Note the amplitudeanomaly extending from about SP-45 to about SP-90 at 1.5 seconds.However, the amplitude anomaly was found to originate from ahigh-velocity conglomerate layer.

Shown in FIGS. 17(a) and 17(b) are the single-fold common-depth-pointgathers at two locations: location A at the well and location B, about1/2 mile to the west. The gathers at location A do indeed indicate theabsence of gas, i.e., no noticeable increase in reflector amplitude withoffset. However, the gathers at location B do show a slight increase inamplitude with offset, i.e., possible gas.

EXAMPLE IV

Seismic data were obtained for another area and are depicted in FIG. 18.The possible gas-related amplitude anomalies are located (i) betweenSP-270 and -310 at about 1.3 seconds and (ii) betwen SP-250 and -300 atabout 1.0 second.

The ten-fold CDP gathers at locations A and B of FIG. 18 are shown inFIGS. 19(a) and 19(b), respectively. Here, there do indeed appear to beindications of amplitude increase with offset. In FIG. 19a, the anomalyappears over a region where amplitude increases with offset. In FIG.19b, the anomaly at 1.0 seconds is thought to be related to low-velocityshale.

EXAMPLE V

Seismic data were obtained for another area and are depicted in FIG. 20.The geologic section was limestone embedded in shale. The gas-relatedanomaly is located over the indicated rectangular area of the Figure.

Here, note that for this lithology, gas is indicated by decreases inamplitude with offset, as shown in FIG. 21 representing CDP gathers atsurface locations 102 and 103 of FIG. 20, as viewed respectively fromright to left in FIG. 21.

The method of the present invention as described provides a geophysicistwith a strong tool for differentiating gas-filled and non-gas reservoirsin a variety of structural combinations, e.g., sand and limestoneembedded in shale. However, the invention is not limited to the abovestructural combinations alone, but is applicable to other anomalouscircumstances as known to those skilled in the art. It should thus beunderstood that the invention is not limited to any specific embodimentsset forth herein, as variations are readily apparent, and thus theinvention is to be given the broadest possible interpretation within theterms of the following claims.

What is claimed is:
 1. A method for increasing resolution of seismicrecords containing high-intensity amplitude events in order to associatesuch events with gas-bearing strata in the earth, comprising the stepsof:(a) generating seismic data, including a record of signals fromacoustic discontinuities associated with said strata of interest bypositioning and employing an array of sources and detectors such thatcenterpoints between selected pairs of sources and detectors form aseries of centerpoints along a line of survey, said recorded signalsbeing the output of said detectors; (b) by means of automated processingmeans, statically and dynamically correcting said recorded signals toform corrected traces whereby each of said corrected traces isassociated with a centerpoint horizontally midway between asource-detector pair from which said each corrected trace was originallyderived; (c) by means of automated processing means, indexing saidcorrected traces in two dimensions whereby each of said corrected tracesis identified in its relationship to neighboring traces on the basis ofprogressive changes in horizontal offset value versus progressivechanges in common centerpoint location, (d) displaying a series of saidtraces of step (c) on a side-by-side basis as a function ofprogressively changing horizontal offset values, said displayed tracesall being associated with at least the same general common group ofcenterpoints so that progressive change in a high-intensity amplitudeevent from trace to trace of said displayed traces is identified as afunction of progressive change in horizontal offset value whereby morelikely than not said event relates to reflections from acousticimpedances with strata containing gaseous hydrocarbons.
 2. The method ofclaim 1 in which step (c) is further characterized by the substepsof:(i) selecting a first series of indexed traces within a commonoffset, common centerpoint window of predetermined dimensions; and (ii)summing said first series of selected traces to form a summed trace. 3.The method of claim 2 with the additional substeps of:(iii) incrementingthe window at least in the common offset dimension to select a secondseries of traces; and (iv) summing the second series of selected tracesto form a second summed trace.
 4. The method of claim 3 in which step(d) is a side-by-side display of said summed traces as a function ofprogressively changing composite horizontal offset values wherebyprogressive changes in said high-intensity amplitude event are moreeasily identifiable.
 5. The method of claim 2 in which the dimension ofthe window of step (i) is four offset values high by five centerpointlocational points long.
 6. A method for converting an originalmultitrace seismic record into an improved section having increasedresolution as to the nature of high-intensity amplitude events relatedto reflections from subsurface strata possibly containing gaseoushydrocarbons, said improved section being composed of a plurality ofamplitude-versus-horizontal offset-and-time traces, said original recordconsisting of a plurality of multitrace seismic traces ofamplitude-versus-horizontal coordinate-and-time, each of said tracesconstituting energy derived in association with a particularsource-detector pair of known horizontal offset and of known centerpointlocation, and representing, in part, event reflections from saidsubsurface strata, said conversion comprising the steps of:(a)classifying said original traces on the basis of common butprogressively changing horizontal offset values and common butprogressively changing common centerpoint locations, whereby each traceis identified by a centerpoint location common to at least another traceand a known horizontal offset value; (b) displaying at least said eachtrace and said another trace associated with said common centerpointlocation, as a function of progressively changing horizontal offsetvalues to form at least a segment of said improved section so thatprogressive change in a high-intensity amplitude event common to saideach trace and said another trace is identified as a function ofprogressive change in horizontal offset, forming said segment of saidimproved section, whereby said event relates to reflections fromacoustic impedances associated with strata containing gaseoushydrocarbons.
 7. The method of claim 6 in which said common centerpointlocation of said another classified trace of step (a) is also common toa plurality of additional other traces, each having known butprogressively changing horizontal values with respect to said anothertrace.
 8. The method of claim 7 in which the step (b) is a side-by-sidedisplay of said each trace, said another trace and said additional othertraces as a function of progressively changing horizontal offset valueto form said improved section whereby progressive change in saidhigh-intensity event is more easily identifiable.
 9. The method of claim7 in which step (a) is further characterized by:(i) indexing all of saidtraces in two dimensions whereby each of said traces is identified inits relationship to neighboring traces on the basis of progressivechanges in horizontal offset value versus progressive changes in commoncenterpoint location; (ii) selecting a first series of indexed traceswithin a common offset, common centerpoint window of predeterminedabsolute dimensions; and (iii) summing the first series of selectedtraces to form a first summed trace.
 10. The method of claim 9 with theadditional steps of:(iv) incrementing the window in at least the commonoffset dimension to select a second series of traces; and (v) summingthe second series of selected traces to form a second summed trace. 11.The method of claim 9 in which step (b) is a side-by-side display ofsaid summed traces as a function of progressively changing compositehorizontal values to form said improved section whereby progressivechange in said high-intensity amplitude event is more easilyidentifiable.
 12. The method of claim 9 in which the window of step (ii)has dimensions of four offset values wide by five centerpoint locationalpoints long.